In the operation of valves for controlling the flow of fluid under pressure, as for example in controlling the flow of feed water to a boiler, cavitation within the valve is always a matter of serious concern. Where excessive cavitation occurs, a throttling valve may be literally destroyed within a few hours of operation. Cavitation results from the fact that, during throttling of a liquid under pressure, the pressure drop of the liquid in the region of the throttling orifices tends to fall momentarily below the vapor pressure of the liquid. In the typical situation, where water is the control fluid, this causes the momentary formation of steam bubbles just downstream of the throttling surfaces. As the flowing liquid continues to travel beyond the throttling surfaces, there is an increase in the pressure to a point above the vapor pressure of the liquid, causing a sudden collapse or implosion of the steam bubbles. This results in a sonic shock wave, which is transmitted through the liquid to the adjacent surfaces of the valve. Where the energy of these shock waves exceeds the fatigue limits of the valve material, the material is quickly cavitated away, destroying the valve in a short time.
In order to minimize cavitation damage it has been proposed heretofore to effect the desired pressure drop in a series of individual steps, so calculated that the pressure drop in any individual step is sufficiently low that cavitation is avoided or greatly minimized. Other proposals involve dividing the flowing fluid into a large plurality of individual streams, such that the cavitation energy of each stream is reduced to an acceptable level. Although these prior proposals have made significant improvements in the operating life of high pressure throttling valves, cavitation remains a serious problem.
In accordance with the present invention, a novel and improved throttling valve is provided in which, in addition to generally minimizing the cavitation energies in accordance with known procedures, the valve is so designed and costructed as to locate the sites of bubble implosion in regions as remote as practicable from the valve parts in order to minimize the cavitation damage resulting from the bubble implosions, which are somewhat unavoidable. In this respect, the cavitation damage resulting from bubble implosion is an exponential function of the distance between the implosion site and the adjacent metal surfaces, such that increasing the distance between valve walls and implosion sites exponentially decreases the effect of the resulting shock wave on the valve parts.
In accordance with one aspect of the invention, a throttling valve in accordance with the above objective is provided with a cylindrical trim cage receiving a controllably positioned cylindrical valve plug. The trim cage is provided with a plurality of radial passages, of special configuration to be described, to permit the radially inward flow of liquid. As the valve plug is retracted from its seat and progressively withdrawn axially through the cylindrical trim cage, increasing numbers of the radial passages are uncovered by the valve plug, permitting progressively increased flow of fluid through the valve. Pursuant to the invention, each of the radial passages is of a configuration such that the effective throttling orifice formed thereby is located as close as practicable to the inner wall surface of the trim cage. In this respect, in accordance with known liquid flow behavior, the liquid both accelerates and contracts its flow stream as it approaches the effective orifice. After passing through the effective orifice, the flow stream continues to accelerate and contract for a predetermined distance, after which it begins to decelerate and expand. The region of lowest pressure of the fluid is the point at which the flow stream is most contracted and traveling at the highest velocity, in other words at the so-called vena contracta. If the pressure in the region of the vena contracta is below the vapor pressure of the liquid, which is often the case in practical applications, bubbles will form. As the flow stream thereafter begins to decelerate and expand, the pressure is increased and the bubbles are recompressed. Thus, the implosion sites of the bubbles typically are located slightly on the downstream side of the vena contracta.
Significant to the invention is an understanding that merely providing that the exit diameter of a radial passage in the trim cage be smaller than any other diameter of the passage will not result in locating the effective orifice at the inner wall of the trim cage. A fluid stream approaching an orifice and beginning to converge and accelerate, tends to form a funnel-like stream, the outer limits of which are of generally parabolic contour. Accordingly, even though the exit opening of the passage is smaller in diameter than any other part of the passage, any upstream portion of the passage that is smaller than the exponential (parabolic) flow contour will prematurely constrict the fluid flow and will serve as the effective orifice. To the extent that such effective orifice is upstream from the inner wall surface of the trim cage, the vena contracta will be formed closer to the wall than desired and, perhaps, even within the wall, so that the resulting implosion downstream of the vena contracta can occur closely adjacent to the valve surfaces, tending to cause excessive cavitation damage.
In accordance with another and more specific aspect of the present invention, a novel and improved throttling valve is provided in which a cylindrical trim cage is formed with a plurality of radial passages, of a generally exponentially converging configuration, yet which is capable of being produced in a practical manner utilizing conventional machining equipment. Toward this end, a theoretically ideal, exponentially converging radial passage is closely approximated by forming the passages in three stages. The innermost stage comprises the primary throttling orifice and is substantially cylindrical in form, of minimum but finite length. The second stage may be of substantially straight-walled conical form joining, at its convergent end, with the orifices section and expanding in a generally radially outward direction therefrom. The third stage of the passage is an arcuate outward flare, which is substantially tangent with the divergent end of the conical section and merges more or less into tangency with the outer walls of the trim cage. By following certain proportions and relationships, a trim cage may be constructed in accordance with the invention utilizing relatively standardized forms of tooling, yet at the same time closely approximating the theoretically desirable exponentially converging radial passage.